In recent years, with the widespread proliferation of cellular phones and portable wireless communication devices, there has been an increased demand for reducing the size and weight of them. The miniaturization of various electronic components including semiconductor integrated circuits has been rapidly advanced. However, the antenna is a hindrance to miniaturization of wireless communication devices. As is well known, the antenna is an entrance and exit for electromagnetic waves and the efficiency of the antenna is extremely low if the antenna is not resonant with a used frequency. In the case of an ordinary dipole antenna, the necessary length is ½ wavelength of a used frequency and it is, therefore, very difficult to reduce the size. Various devices relating to miniaturization of antennas have therefore been proposed.
For example, an antenna disclosed in Japanese Patent Laid-Open No. 10-13135 is formed so as to be smaller in size and capable of resonating in two frequency bands by having an antenna element folded back so as to be substantially parallel along the longitudinal direction.
Also, an antenna disclosed in Japanese Patent Laid-Open No. 10-229304 is devised in such a manner that an antenna element is formed on a surface of a dielectric substrate to enable the antenna to be further reduced in size and to be used by being mounted on a circuit board in a simple manner.
However, with the advancement of the miniaturization and integration of cellular phones and portable wireless communication devices, a need arises for placement in the vicinity of a grounding conductor pattern formed on a circuit board at the time of mounting on the circuit board. In such a case, there is a problem that if a conventional antenna is placed in the vicinity of the grounding conductor pattern, the resonance frequency of the antenna is changed so that the VSWR in a frequency band used for communication is increased and the efficiency is considerably reduced.
For example, in the case of a 2.4 GHz dielectric antenna in the form of a rectangular block, if it is necessary to place the dielectric antenna in the vicinity of a grounding conductor due to the above-described advancement of the miniaturization and integration, a voltage standing wave ratio (hereinafter referred to as VSWR) required to enable use of the antenna cannot be obtained unless the distance between the dielectric antenna and the grounding conductor is increased to a value equal to or larger than 3 mm.
On the other hand, while internet connection (dial up connection) through a cellular phone connected to a notebook-type personal computer away from home has been used, internet connection using a kind of wireless communication is now attracting attention. This is a service generally called “hot spot”, i.e., a system in which a wireless LAN base station is installed in a certain building and internet connection is established therethrough. Frequencies in the 2.4 GHz and 5.2 GHz bands are used for this wireless LAN.
Therefore there is a need to provide an antenna capable of communication in two frequency bands in a case where a wireless communication device for use with a wireless LAN capable of communication in the two frequency bands is configured. Thus, there is a hindrance to miniaturization of wireless communication devices.
In view of the above-described problem, an object of the present invention is to provide a small dielectric antenna capable of obtaining a good VSWR characteristic in frequency bands to be used even if the antenna is placed in the vicinity of a grounding conductor pattern when mounted on a circuit board. Another object of the present invention is to provide a small dielectric antenna having a good VSWR characteristic in two different frequency bands.